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Synthesis and Regulation of Thyroid Hormones

Synthesis and Regulation of Thyroid Hormones

Low blood levels of the thyroid hormones — triiodothyronine (T3) and thyroxine (T4) — signal the hypothalamus to release the thyrotropin-releasing hormone (TRH). TRH then reaches the pituitary gland and stimulates the release of thyroid-stimulating hormone(TSH) into the bloodstream.
Upon reaching the thyroid gland, TSH stimulates the follicular cells' active uptake of iodide ions from the blood. The ions diffuse to the apical surface of the cells and are oxidized to iodine. The iodine is then...

Functions of Thyroid Hormones

Functions of Thyroid Hormones

The thyroid hormone (TH) plays a pivotal role in the intricate orchestration of physiological processes, exerting profound effects on development, metabolism, and homeostasis throughout different life stages.
TH is indispensable for the normal development and maturation of the skeletal, muscular, and nervous systems during fetal and childhood growth. It facilitates bone mineral turnover and regulates protein synthesis in developing tissues, contributing significantly to overall growth and...

Hyperthyroidism II: Pathophysiology

Hyperthyroidism II: Pathophysiology

Hyperthyroidism is a hypermetabolic state caused by elevated levels of thyroid hormones, triiodothyronine (T3) and thyroxine (T4). It results from dysregulation at the thyroid, pituitary, or immune system level and affects multiple organ systems.PathophysiologyThe most common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder in which antibodies, specifically thyroid-stimulating antibodies (TSAb), a subtype of TSH receptor antibodies (TRAb), bind to and activate TSH receptors...

Hyperthyroidism I: Introduction

Hyperthyroidism I: Introduction

Hyperthyroidism is a type of thyrotoxicosis characterized by the thyroid gland's overproduction of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). This hormone excess increases the basal metabolic rate and enhances sensitivity to catecholamines.DiagnosisDiagnosis is based on clinical features and biochemical testing. It typically shows suppressed thyroid-stimulating hormone (TSH) levels below 0.4 mIU/L, with elevated free T3 and/or T4. Additional tests, including thyroid...

Hypothyroidism II: Pathophysiology

Hypothyroidism II: Pathophysiology

Hypothyroidism is a disorder characterized by insufficient production of thyroid hormones, which regulate metabolism, energy balance, and multiple organ systems.TypesHypothyroidism is classified based on the level of dysfunction. Primary hypothyroidism results from intrinsic thyroid gland dysfunction, causing reduced hormone production despite normal or increased stimulation. Secondary hypothyroidism arises from inadequate thyroid-stimulating hormone (TSH) secretion by the pituitary. Tertiary...

Hormonal Control of the Ovarian Cycle

Hormonal Control of the Ovarian Cycle

The ovarian cycle is meticulously regulated by the hypothalamic-pituitary-gonadal axis. This cycle orchestrates the release of a mature oocyte, essential for reproduction.
Before puberty, the hypothalamus releases GnRH in a low frequency, low amplitude pulsatile manner. This along with the immature hypothalamic-pituitary-gonadal axis activity, results in low estrogen levels and the absence of a fully functional ovarian cycle. At puberty, GnRH secretion increases in both frequency and...

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Relationship between thyroid function and seasonal reproductive activity in mares.

G Huszenicza¹, P Nagy, J Juhász

¹University of Veterinary Science, István u. 2., 1078, Budapest, Hungary.

Journal of Reproduction and Fertility. Supplement

|August 5, 2010

Summary

Thyroid hormones triiodothyronine and thyroxine are lower in anestrous mares during the non-breeding season. These changes may be influenced by hypothalamic control, not direct thyroid gland involvement in equine seasonality.

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Blood Sampling and Hormone Measurement for Determining the Stage in the Ovarian Cycle in Marmosets

Blood Sampling and Hormone Measurement for Determining the Stage in the Ovarian Cycle in Marmosets

Published on: July 5, 2024

Area of Science:

Reproductive Endocrinology
Equine Physiology
Thyroid Function

Background:

Seasonal reproductive activity in mares is influenced by environmental factors.
The role of thyroid hormones in equine reproductive seasonality requires further investigation.

Purpose of the Study:

To investigate the relationship between thyroid function and seasonal reproductive activity in mares.
To compare plasma triiodothyronine and thyroxine concentrations in anestrous and cyclic mares during the anovulatory season.

Main Methods:

Plasma concentrations of triiodothyronine and thyroxine were measured in anestrous and cyclic mares.
Hormone levels were monitored in mares that foaled before the vernal equinox.
Thyroid hormone concentrations were assessed in adult and young mares.

Main Results:

Significantly higher thyroxine concentrations were observed in cyclic mares compared to anestrous mares.
Lower plasma triiodothyronine and thyroxine concentrations were found in anestrous mares post-foaling and during the anovulatory period.
Increases in thyroid hormones correlated with the first ovulation post-parturition.

Conclusions:

Decreased thyroid hormone concentrations in anestrous mares may be a result of hypothalamic control, not direct thyroid gland regulation of seasonality.
Thyroid function does not appear to be the primary driver of seasonal reproductive activity in horses.